In the stabilization of a friction soil and an adjacent column soil layer, it is known from EP-B 0 203 137, for example, to sink a compaction probe of substantially constant longitudinal cross section through the soil strata utilizing a vibrator attached at an upper end to the compaction probe. The latter, which can be a beam or bar, is subsequently extracted from the soil by vibration.
Building loads can be transferred from the ground surface to underlying soil or rock strata by the use of pile foundations. The piles which are used for this purpose are driven into the ground by boring, ramming or vibrating.
In many cases the piles are loaded to their maximum extent during the driving phase and it is this brief phase which is critical for pile design since long-term loading by the building may not place as much stress upon the pile.
Pile foundations, therefore, are most economical in the case of highly concentrated building loads. In more frequent cases, however, the loads are lighter and for residential and industrial buildings of medium height, for embankments and the like, the load bearing capacity of piles is not fully utilized. In such cases sufficient bearing strength can be obtained by soil stabilization techniques.
When one refers to soil stabilization, one must distinguish between fine-grained soils, so-called cohesive or cohesion soils, and coarse-grain, water-permeable soils which are referred to as friction soils.
The loading of fine-grain soil can result in settlement only after long periods of time, measured in years. Settlement can occur in coarse-grain or friction soils in much shorter periods of time measured in minutes or days. The different soil properties have had a significant influence on the choice of the soil stabilization method.
The various soil stabilization methods which have been used in the past have been developed in response to various requirements. In friction soils, for example, driving, vibrating or oscillating methods are primarily employed. The increased bearing capacity of these soils results from the dynamic forces applied as for example in the case of a vibrofloatation technique or resonance compaction. In the resonance compaction approach, the specially designed compaction probe or bar is vibrated vertically into the soil and the vibration frequency of the vibrator attached to the bar is set to a resonance frequency of the soil deposit to achieve the most effective soil compaction.
The strength of fine-grain soils, such as silt or fine sand, can be improved by adding material of greater bearing capacity like coarser sand or gravel, and simultaneously imparting a mechanical treatment by ramming or vibrating for example.
The result may be pillar-like columns of sand or gravel often required to as "stone" columns which, however, have limited bearing capacity.
To improve soils to great depths by this method, specialized machines have been developed which can penetrate into the soil to be improved by vibration flushing or other mechanical procedures involving pushing or screwing in order to produce the stabilized columns.
In cohesive soils such as clays, oscillating or vibrating methods are not applicable. In such cases it is possible to mix into the soil, certain stabilizing substances like cement, fly ash or lime which react with the surrounding soil and can set to produce stabilized soil columns. This method becomes significantly more expensive with increasing depth and can only be utilized with certain fine-grain soil types.
It is also possible to improve fine-grain soils by installing vertical drain elements in the soil. These elements are usually not load bearing or even soil stabilizing elements, being of insufficient stiffness. They do serve to increase the permeability of the soil in order to dissipate water and reduce the water pressure in the interstices of the soil. Drainage improvement, therefore, most often must be combined with other methods, like static preloading which increases the rate of soil settlement, after which a structure can be applied to the ground. While this method is of relatively low cost, it is very time consuming and is not practical in many cases. The drain structures can be composed of coarse grained material like sand, waste products from industry like gypsum or fly ash or from synthetic materials such as plastics and reinforced cardboard. The drain elements can be installed in the soil by pushing, vibration, driving, flushing or a combination thereof.
In construction, mixed soil deposits frequently are encountered. Such mixed deposits can consist of both fine grain and coarse grain soil strata. As a result it has been difficult in the past by any single method to provide a technological satisfactory and economical ground stabilization. For example, when one uses the vibroflotation method, sand layers can be effectively stabilized but intermediate layers of clay or silt are not positively affected by the method.
It may be mentioned that a new approach to soil stabilization, referred to as soil nailing, has been developed for the stabilization of slopes and excavations. In this method, long stiff elements (rods or soil nails or concrete) are driven or bored into the soil. The soil nails are installed with a small spacing from one another (about 0.5 to 1.5 m), i.e. a spacing much smaller than that used with piles in the formation of pile foundations. The soil nails, without providing a material load supporting effect in themselves, impart to the soil a reinforcement enabling the building loads to be carried by the reinforced soil primarily and only partially by the soil nails themselves.
This technique has not been widely used to solve foundation problems occurring with other approaches primarily because of the difficulty of installing safely, with precision and with the requisite closeness of spacing of the long but slender nails which can have diameters of 2 to 200 mm and lengths of up to 20m.